Method of making a prestressed heat exchanger



R. A. ARES 3,457,620

METHOD OF MAKING A PHESTRESSED HEAT EXCHANGER Filed March 23, 1967 F 7 mm 4 EA 1 m M l 4 x 0 m .M m n P Y B Z 0 2 4 r- M l -w H I l )(J E 4 l I E k WMWH Wm M M n LMHWH H M F 4 m r r I m u v.

' Jul 29, 1969 FIG-E PIE .4

United States Patent 3,457,620 METHOD OF MAKING A PRESTRESSED HEAT EXCHANGER Roland A. Ares, 816 Cascade Road, Wilmington, N.C. 28401 Filed Mar. 23, 1967, Ser. No. 625,401 Int. Cl. 1323 15/26 U.S. Cl. 29-157.3 4 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a method of forming a finned tube steam coil. Illustratively, the heat exchanger may be of the type which comprises a rectangular frame and a single row of fluid tubes extending therein, said tubes having plate-type fins secured perpendicular thereto for transfer of heat between relatively hot gas or fluid flowing through the tubes and relatively cold air or other gas passing across the fin surfaces. Such heat exchangers are particularly useful in air conditioning arrangements.

The principal features of the present invention resides in: (1) a method of forming the heat exchanger wherein one end of each fluid tube is anchored in a common frame portion, and an expanding tool is simultaneously run through each of the tubes from the other end in a manner to slightly bow the rigidly assembled frame tube sheets through tube prestressing; and (2) a prestressed tube coil allowing the fluid or gas transmission tubes to undergo considerable thermal expansion in service without breaking or rupturing through utilization of the frames tube sheet flex. Said flex is resultant of the tube sheets concave due to tube prestressing.

CROSS REFERENCES TO RELATED APPLICATIONS There are no other known patent applications which have any particular relation to the present application.

BACKGROUND OF THE INVENTION The invention is particularly applicable to the manufacturer of steam to air extended surface coils used in heating applications, to wit plate fin steam coils.

There is no known prior art patent relating to this instant type of (1) manufacturing process and (2) end use thereof.

SUMMARY This invention contemplates a method of manufacturing a heat exchanger coil wherein a plurality of apertured plate fins are loosely positioned in a perpendicular attitude on a row of fluid tubes. The tubes are located between a pair of tube sheets forming part of a rectangular framework; thereafter one end of each tube is anchored in one of the tube sheets, and an expander tool is run through each tube from its non-anchored end. As the expander tool begins its travel in the tubes it causes the other tube sheet to be anchored to each tube; subsequent movement of the expander tool causes a radial expansion of the tube with a consequent axial contraction, thereby slightly bowing in the two tube sheets and applying a prestress onto the tubes. The prestressed tubes can undergo considerable thermal expansion, normally encountered in frequent on/ off duty cycles, during service without fracture or de- Patented July 29, 1969 THE DRAWINGS FIG. 1 is a longitudinal sectional view taken through a. partially fabricated heat exchanger made by the method of this invention.

FIG. 2 is a fragmentary view of an expander tool used with the FIG. 1 heat exchanger.

FIG. 3 is a right end view of the FIG. 1 heat exchanger.

FIG. 4 is a longitudinal sectional view of the FIG. 1 heat exchanger taken at a later stage in the manufacturing process.

FIG. 5 is an enlarged sectional view of a tube to tube sheet initial anchoring joint in the FIG. 1 heat exchanger.

FIG. 6 is a fragmentary sectional view taken on line 6-6 in FIG. 4.

FIG. 7 is an enlarged fragmentary sectional view illustrating the mode of operation of the FIG. 2 expander tube.

FIG. 8 is a fragmentary sectional view showing a header commonly employed with the FIG. 4 heat exchanger.

GENERAL ARRANGEMENT FIG. 1 shows a partially formed heat exchanger comprising a rectangular framework 10, a series of cylindrical fluid tubes 12 supported in the opposite end walls of the framework, and a plurality of plate-type heat transfer fins 14 carried by the fluid tubes. In service the rectangular framework may be bolted or otherwise positioned within an air duct or heating unit so that air passes through the spaces between the heat transfer fins. High temperature steam or hot water may be passed through tubes 12 to conduct heat to the fins 14 and thereby transmit heat the air passing across the tin surfaces. Normally inlet and outlet headers or serpentine return bends (not shown in FIG. 1) are connected with tubes 12 to supply fluid (e.g. steam or hot water) to each tube.

Framework 10 comprises a first tube sheet 16, a second tube sheet 18, a first longitudinal tie element 28 and a second longitudinal tie element 22. The two tube sheets 16 and 18 are similar to one another, and the two tie elements 20 and 22 are similar to one another. Each tube sheet is of channel cross section comprising a web Wall 24 and two flanges 26 and 28. Additional flanges 30 and 32 are formed by out-turned upper and lower edge portioas of the web wall 24. The tube sheet may be formed of various gage metals, as for example 16 gage galvanized steel, and with various turned ed ges.

Each tie element 26 and 22 is of channel configuration comprising a web wall 34, and flanges 36 and 38 having inturned ends 40, but neither necessarily restricted to channel and inturned configuration. Securement of the two tie elements "20 and 22 to the tube sheets 15 and 18 may be accomplished by multi nut-bolt assemblies 42, arranged in multiple pairs, one pair in each corner of the framework. The bolts of course go through the web walls 34 of the tie elements and flange walls 39 or 32 of the tube sheets. The formed framework is thus rigid at its corners; additionally the semibox nature of each tie element 20 or 22 makes each tie element rigid and substantially buckle-free along its entire perimeter. Each tie element may be formed for example of 16 gage galvanized steel, but not necessarily restricted to 16 gage galvanized steel.

IIllustratively, each of the aforementioned fluid transmitting tubes 12 may be formed of inch 0. D. copper tubing having a wall thickness of .020 inch. Each platefin 14 is preferably an aluminum plate having a Wall thickncss of about .010 inch; the fins may be spaced from one another about .100 inch, making about ten fins per inch. As shown in FIG. 7, each fin may be provided with integral collars 44 for its correct spaced positionment on fluid tubes 12.

ASSEMBLING THE HEAT EXCHANGER COMPONENTS The heat exchanger may be fabricated in different sequential steps, but for illustration purposes we will assume the initial step is to anchor tubes 12 to tube sheet 18; this may be accomplished before tube sheet 16 has been attached to tie elements 20 and 22. As shown best in FIG. 5, each tube 12 may be anchored pinned to sheet 13 by rolling the tube material outwardly in the tube areas adjacent opposite faces of the tube sheet. The rolling operation can be performed by inserting a conventional off-center roller mechanism into the tube and thereafter rotating said inserted tool a minimum of one complete revolution to outwardly fashion the tube wall to its FIG. configuration. Alternately each tube 12 can be secured to sheet 18 by tack welding or pressure clamping procedures.

Before or after one end of tubes 12 have been anchored in tube sheet 18 the tube 12 may be threaded into the apertured fins 14. Thereafter tube sheet 16 may be connected to longitudinal tie elements 20* and 22 to form the assembly of FIG. 1.

Final fixed connection of tubes 12 with tube sheet 16 and fins 14 may be accomplished by running or driving an expander tool 50 internally through each tube 12, beginning from the free or non-anchored end and terminating at the anchored end. As the tool moves through each tube it initially expands the tube into tight gripping engagement with edge surfaces of the openings in tube sheet 16, thus causing the tube to be anchored to both tube sheets 16 and 18 (since each tube has already been anchored to sheet 18). Subsequent internal tube movement of tool 50 causes the tube to be radially expanded as shown in FIG. 7, whereby producing required prestressing of the tube through consequential axial contraction due to said tube radial expansion. The aforementioned tube radial expansion also produces the prior art tube and fin positive mechanical bond needed for promoting good thermal conductance.

For illustration purposes the deformation of tube 12 is exaggerated in FIG. 7. In an actual construction, using a tube 12 having an inner diameter of .585 inch and a wall thickness of about .020 inch, the expander tool would have a diameter of about .598 inch so that the tube internal surface would expand about .0065 inch on a radial basis. The fins, spaced about .1 inch and formed with a wall thickness of about .01 inch, would initially have a sliding fit on tube 12; the .006 inch radial expansion of the tube would be sufficient to take up any play or looseness between the fin collars and tube surface thus mechanically bonding the two together.

EFFECTS OF TUBE EXPANSION As tool 51 radially expands tube 12 it also contracts the tube axially. The radial movement is thus achieved partly by stretching the tube material outwardly and partly by drawing the tube material longitudinally. The reduction in length of each tube 12 incident to a complete traverse of tool 50 is not large, being on the order of 36% This reduction in tube length is however sufiicient to slightly bow in the tube sheets 16 and 18 as shown in FIG. 4. The actual magnitude of the bow-in would be less than that pictured in FIG. 4, but would be sufficient to com pensate for thermal expansion of the tubes during service.

Working with tubes 12 having lengths of inches, the thermal expansion of a copper tube subjected to a temperature change of approximately 300 F. would be about .217 inch. The bow-in of tube sheets 16 and 18' which accompanies a complete traverse of tool 50 is about .3 inch. Thus the bow-in is sufiicient to compensate for thermal expansion of the tubes during service.

Another result of the tube anchoring and consequent tube prestressing is the reduction of the expanded tube initial wall thickness within the collared fin region only. This reduction is on the order of about .O0=l inch but is still able to withstand the design work pressure due to the full fin collar 44 bridge effect. This wall reduction affords the low order 36% shrink factor, normal shrink in conventional construction is a 2% factor. Being at a lower magnitude, .36 vs. 2, less tubing is shrunk (for original tube wall is not being endeavored to be maintained) resulting in less tubing being used.

In further explanation of the bow-in action, it should be noted that as soon as tool 50 has caused tubes 12 to be anchored to tube sheet 16, subsequent movement of tool 50 to axially contract the tubes will be opposed by the two longitudinal tie elements 20* and 22. The tie elements will be put in axial compression but will not buckle due to their box-like cross sections; if they were formed of less rigid cross sections external reinforcements could be provided to prevent buckling or bowing. Assuming no appreciable buckling or bowing of tie elements 20 and 22, the axial contraction of each tube 12 will produce an inward bowing of the tube sheets 16 and 18 as represented generally in FIG. 4. As shown in FIG. 6, there will also be a slight buckling of each tube sheet along the minor axis of the heat exchanger.

In actual use of the described process either a single or multi expander tool 50 may be employed for tube subsequent prestressing. It is believed more economically desirable to accomplish tube expansion with a bank of expander tools 50 simultaneously movable through all of the tubes or selected groups of tubes simultaneously. Preferably the anchored end of each tube (right end in this case) is abutted against a stationary expander bed or socket group (not shown) to maintain each tube in a fixed position during the tube expansion process. Frame 10 is preferably maintained in an unrestrained or floating condition during the tube expansion process.

DIFFERENTIATION OVER PRIOR PRACTICE The described manufacturing process differs from known processes primarily in the concept of anchoring each tube 12 only at one of its ends prior to running tool 50 through the tube; this accomplishes the aforementioned tube prestressing, resulting in bow-in action of tube sheets 16 and 18. In conventional processes tubes 12 would be first fitted into tube sheet 18, subsequently threaded through fins 14, then through tube sheet 16. The tubes would then be expanded by tool 58 thus anchoring the tube 12 first to tube sheet 18, then fins 14, then tube sheet 18. Thus, a complete unit finned tube subassembly would be formed apart from the longitudinal tie elements. This subassembly would then be inserted into the framework during framework fabrication. The tube sheets 16 and 18 would thus not undergo a bowin action during tube expansion nor would tubes 12 be prestressed. Also the fixed framework would not be in evidence about the pre-expanded assembly dictating more cautious and elaborate handling procedures.

Conventional methods of manufacture of such similar end use extended surface plate fin coils is by use of excepted coil expansion methods, to wit the longitudinal tie members are not preaffixed to the tube sheets, nor is the tube preanchored at one of the tube sheets. The resultant tube expansion, for good thermal bond between the tube and the fin, produces the normal 2% tube shrinkage. The in-service thermal expansion was intended to be expanded either through utilization of sliding core or oversize tube sheet tube holes. The sliding core employs the expanded tube-fin core in an oversize rigid box-like channeled frame, completely enveloping the core at the top and bottom and side, which permits floating movement during duty service. The oversize tube sheet tube holds employ oversize tube holes in one of the tube sheets, i.e. allowing for tube service expansion and contraction at one coil end by not fastening one of the tube sheets to any tubes.

Using conventional techniques the tubes 12 undergo considerable axial contraction during the tube-expansion process. Thus, a 90 inch finned copper tube of the construction previously described (.585 inch I.D., expander tool diameter of .598 inch, .01 inch collared fins spaced .1 inch) undergoes an axial contraction of about 1.8 inches when it is expanded prior to being anchored in the frame. It appears that use of the proposed preanchorage concept causes the tube wall thickness to be slightly stretched during the expansion process. This is not particularly detrimental strength-wise since the fin collars reinforce the tube along its entire length. The lessened axial contraction (.36 inch compared to 2 inches) is however beneficial in reducing the length thus cost of tubing required per given size coil.

SIZE AND OPERATION VARIATIONS FIGS. 1 and 4 show a heat exchanger having one row of six fluid tubes. This is primarily for illustration purposes to best show the tube-fin relationship. An actual prestressed heating coil would in most cases have more tubes, as for example twenty. The prestressed heating coil can be formed of different dimensions or sizes, as for example tube lengths varying from 12 inches to 90 inches and tube sheet lengths varying from 8 inches to 40 inches. The invention has at present been practiced only on single row tube constructions, although conceivably the process could be applied to multiple row arrangements. The tube diameters, tube lengths, configurations, materials, etc. can be varied within wide limits while still practicing the invention.

Tubes 12 can be connected into a fluid system by any suitable types of headers or serpentine circuited by the use of return bends and two open tube connection points. for illustration purposes there is shown in FIG. 8 a tubular header 54 equipped with a supply fitting 56 for introducing steam or other high temperature fluid to the various tubes 12. An internal baflle 58 may be provided to assist distribution of the fluid equally to all of the tubes. Additionally, presized orifice plugs (not shown) may be positioned in each tube 12 supply header end to provide desired fluid flow in each tube.

In the previous description tubes 12 were stated to be inserted into the apertures in tube sheet 18 then threaded into fins 14 for a fin-tube core. It will be understood however that fins 14 could be strung on tubes before connecting or anchoring the tubes to tube sheet 18. Tie elements 20 and 22 could if desired 'be connected with tube sheet 18 after anchoring tubes 12 onto sheet 18. As a still further alternative the frame could be completely farbicated, the fins located within the frame by suitable fixturing, and the tubes then inserted through the apertures in the fins and tube sheets; the final steps would then consist of anchoring or pinning the tubes to sheet 18 and expanding the tubes by means of tool 50. Whatever the exact sequence of assembly operations, it is essential under this invention that one end of each tube 12 'be anchored in one of the tube sheets before running the expander tool 50 through the tubes. Also, it is essential that the tool be pushed or pulled through the tube from the non-anchored tube end toward the anchored end; this achieves the desired anchoring of both tubes on tube sheet 16 prior to axial contraction of the tubes.

It is claimed:

1. A method of forming a prestressed extended surface heating coil comprising the steps of positioning a plurality of apertured fins loosely on a row of fluid tubes; building a rectangular framework by securing the ends of two apertured tube sheets to two interconnecting longitudinal tie elements; locating the tubes with their end portions projecting through selected apertures in the respective tube sheets; anchoring one end only of each tube by aflixing same to one of the tube sheets; and thereafter affixing each tube to the other tube sheet then to the fins; said affixing step being performed by running an expander tool internal through each tube, beginning from the end thereof which is not anchored and terminating at the anchored end, thus prestressing through tube wall expansion and subsequential tube longitudinal contraction.

2. The method of claim 1 wherein the step of locating the tubes with respect to the tube sheets is performed before the tube sheets are secured to the tie elements.

3. The method of claim 1 wherein the step of securing the tube sheets to the tie elements is performed before the tubes are located in the tube sheet apertures.

4. The method of claim 1 and further comprising the step of restraining the tie elements against bowing outwardly during the step of running the expander tool through each tube.

References Cited UNITED STATES PATENTS 1,646,385 10/1927 Bergstrorn 29---202 X 2,023,738 12/1935 Mason et a1. 29202 3,292,689 12/1966 Kimura 29l57.3 X

JOHN F. CAMPBELL, Primary Examiner D. C. REILEY, Assistant Examiner U.S. Cl. X.R. 29-202; ll3-ll8 

